1. Field of the Invention
The present invention is related to the field of geophysical exploration and more specifically to a system and method for synchronizing downhole and surface-acquired data.
2. Description of the Related Art
A seismic receiver typically is deployed in a wellbore for determining the response of the earth to seismic energy in the vicinity of the wellbore, which enables determination of certain characteristics of the earth in the vicinity of the wellbore, such as geological structure and the location of changes in the material properties of the earth which may naturally occur.
One of the reasons for using a borehole seismic receiver is for matching various depths within the earth penetrated by the wellbore to specific travel times of seismic energy generated at the earth's surface. In relatively unexplored areas, geophysical surveys are typically conducted entirely at the earth's surface. Being able to determine the time for seismic energy to travel to a particular depth within the earth using a surface seismic survey depends on a portion of the seismic energy generated at the earth's surface for the survey being reflected from a zone in the earth having an acoustic impedance mismatch. Impedance mismatches, known as reflectors, typically occur at boundaries of changes in material composition or material properties of the earth.
Reflectors are of particular interest for identifying possible exploration targets within the earth. Each reflector has associated with it a seismic travel time, determined in the surface seismic survey. In order to calculate the depth to a particular reflector, it is necessary to determine the velocity of the seismic energy through the earth. The velocity of the seismic energy through the earth is strongly related to the composition and material properties of the earth. The material properties of the earth may vary widely within different earth formations within the depth range traversed by the wellbore.
It is difficult, if not impossible, to explicitly and accurately determine the seismic velocity of formations solely from the surface seismic survey, therefore when a wellbore is drilled in a relatively unexplored area, a borehole seismic receiver is used to make measurements to determine the velocity of the seismic energy within the formations.
Determining the velocity of the formations while the wellbore is being drilled, rather than after the drilling is completed, can be particularly valuable in certain instances. For example, some wellbores are drilled directionally to the exploration target because the target is horizontally displaced from the location of the wellbore at the earth's surface. If the target was selected only on the basis of seismic travel time to a reflector, then the depth to the target may not be precisely determinable without knowing the velocity of the formations from the earth's surface to the target. This lack of knowledge could cause the planned wellbore trajectory to miss the target entirely.
Periodic use of a wellbore seismic receiver during drilling in conjunction with a seismic energy source deployed at the earth's surface directly above the position of the wellbore seismic receiver enables measurement of seismic energy travel time to the depth of the seismic receiver deployed in the wellbore. The measurement of seismic travel times to various depths enables calibration of the surface seismic survey travel time in depth, thereby increasing the probability that the wellbore will penetrate the target.
Certain reflectors observed on the surface seismic survey are of particular concern in drilling the wellbore. For example, reflectors sometimes correlate to the presence of significant changes in the gradient of fluid pressure contained within some formations. Knowledge of the precise depth of the reflector could prevent drilling problems which might result from unintended penetration of a formation containing fluid pressure with a significantly different gradient than the gradient otherwise expected. The use of a borehole seismic receiver to calibrate seismic travel time to the wellbore depth could enable more precise determination of the depth of the reflector, which could prevent unintended penetration of formations having abnormal fluid pressures.
It is also known in the art to use borehole seismic receivers for generating seismic reflection sections in an area around the wellbore. Seismic energy from the seismic energy source also travels deeper than the receiver in the wellbore and can be reflected by deeper zones having acoustic impedance mismatch, just as with a surface seismic section. The reflection energy can be identified by appropriate processing of a recording of the energy detected by the receiver. The identified reflection energy can be displayed in a form for comparing the borehole seismic survey with the surface seismic survey.
Systems and tools are known in the art for detecting and storing seismic signals downhole for retrieval and processing on the surface. U.S. Pat. No. 5,555,220 to Minto, assigned to the assignee of this application and incorporated herein by reference, describes a seismic receiver deployed to the bottom of a drill string on a slick line for taking seismic survey data. Seismic data is received and stored and the receiver is retrieved to the surface. A clock in a surface controller is synchronized with a clock in the deployed receiver. The source data is time-stamped using the surface clock. The received data is time-stamped using the downhole clock. The accuracy of the resulting seismic profile is dependent upon the accurate synchronization of the clocks. The downhole clock, in particular, is susceptible to drift caused by substantial changes in temperature found in the downhole environment. The two clocks typically require synchronization of 1-2 milliseconds or better to achieve acceptable profile accuracy.
Another such system is that described in U.S. patent application Ser. No. 10/108,402 to Jackson, assigned to the assignee of this application, and incorporated herein by reference. Jackson describes a method for deploying a seismic receiver in a drill string by dropping and/or pumping the receiver to the bottom where it is latched to the drill string. Seismic signals are received, time-stamped by a downhole clock, and stored in memory in the receiver at multiple predetermined locations during the tripping of the drill string out of the hole. The signals are retrieved at the surface and combined with surface source data that has been time-stamped by a surface clock. Again, the accuracy of the resulting profiles rely on the synchronization of the surface and downhole clocks.
Typical deployment times for the above-described tools is 12-48 hours. This fact translates into a need for clock stability better than 1×10−8 over the deployment time. Downhole clocks commonly use piezoelectric crystal oscillators that tend to drift with temperature and age. Such clocks are also susceptible to errors caused by shock and vibration during deployment. Using the best techniques known in the art, downhole clocks rarely exceed a stability of 1×10−7. The downhole clock drifts out of synchronization with the surface clock, causing unacceptable degradation of the output seismic profile data.
Seismic measurements may also be made with measurement while drilling (MWD) systems, also known as logging while drilling (LWD) systems. In such applications, the deployment time may be hundreds of hours, exacerbating the problem of clock drift. Several re-synchronizing techniques have been proposed, however these techniques are not always operationally acceptable and/or successful.
There is a need for a downhole clock that is resistant to operationally-induced error and drift for use in downhole systems including downhole seismic systems.